Students are required to study six subjects, including a second language, maths, science, arts and a humanities subject, plus a compulsory philosophy subject called "Theory of Knowledge".

They must also write a 4000-word research essay and complete about 150 hours of community service and physical and creative activities known as CAS (Creativity, Action, Service).

Lauriston Girls' School students Emily Chan, Kimberley Chow and Georgina Davie obtained the perfect score of 45, equivalent to an Australian Tertiary Admission Rank of 99.95.

Emily, who will learn tonight if she has been accepted into law at Cambridge University, said she really enjoyed the extra aspects of the IB diploma, although she admitted they challenged her time-management skills.

For the CAS component of the diploma, which Emily described as "essentially the same as the Duke of Edinburgh Award", she was concert master for her school orchestra, ran, and performed the violin at a nursing home each week.

She also focused on music for her 4000-word extended essay, examining the romantic characteristics of Grieg's Violin Sonata No. 3 in C minor.

"The actual topic was how the sonata strayed from the traditional classical characteristics of the sonata-allegro form," Ms Chan said.

"I'd played that piece for one of my violin exams last year so it was really good to further explore the historical contextual aspects of the work."

Ms Chan said one of the biggest challenges of last year was cutting her extended essay down to 4000 words. "A week before it was due I still had 9000 words, so I was quite frantic. That was a very hectic time of the year because compositions for music were due at the same time."

Ms Chow, who is hoping to study either biomedicine or science at Melbourne University, said the decision about whether to do VCE or IB was "one of the biggest dilemmas" faced by girls at Lauriston in year 10.

"It is just one of those decisions you make and see where it takes you," she said.

Ms Chow, whose favourite subject was biology, analysed the effects of smoke on seed germination for her extended essay.

She chose life drawing, running and volunteering at the Friday Night School, a tutoring program for migrant and refugee children, for her CAS component.

In addition to the three perfect scores, 93 per cent of the 41 Lauriston students who did the IB diploma received at ATAR equivalent of 90 or above.

Presbyterian Ladies' College also excelled with four of the 32 IB students obtaining the perfect score and and another four scoring 44/45.

Principal Elaine Collin said the IB students impressed her every year with their commitment and perseverance.